a) Field of the Invention
The present invention relates to a lead-free tin-silver-based soldering alloy, and more particularly, to a soldering alloy which has a low melting point equal substantially to alloy H, which is a general lead-free tin-silver-based soldering alloy, and is excellent in mechanical characteristic, that is, in a tensile strength, an elongation value and heat resistant fatigue characteristic.
b) Description of the Prior Art
Conventionally, lead has been contained in a soldering alloy. In recent years, the lead has eluted from wasted IC chips, printed circuit boards or the like and, as a result, the groundwater has been contaminated. For this reason, there has arisen an environmental problem such as lead poisoning or the like. Under such circumstances, the study and development of a lead-free soldering alloy have been made, and then, various lead-free soldering alloys have been proposed. In general, alloy H (manufactured of Nippon Alpha Metals Co., Ltd.) has been well known as a lead-free soldering alloy which is used in the aforesaid IC chips, printed circuit boards, etc. Alloy H consists essentially of Ag: 2.0% by weight, Cu: 0.5% by weight, Bi: 7.5% by weight and a balance being Sn. Further, alloy H has a melting point of 212.degree. C., which is higher than a Pb-Sn eutectic solder, and has a melting point lower than a conventional silver-tin-based soldering alloy which does not contain Pb, Cd or the like, and has excellent melting characteristics. However, in alloy H, the elongation value is low in a tensile test and there exists a low temperature eutectic phenomenon. Further, if alloy H is kept at a high temperature, the elongation of alloy H deteriorates and, for this reason, alloy H has inferior heat fatigue characteristics. Further, alloy H cannot absorb the thermal expansion difference between the printed circuit board and components when a temperature cycle is applied to a soldering portion and, for this reason, there is the possibility that the soldering portion fractures.
Inventors of the present invention have made various studies in order to find a soldering alloy which does not contain harmful lead or the like and has an excellent melting characteristic, tensile strength and elongation value. Further, the inventors have previously proposed the invention (alloy) disclosed in Japanese Patent Application Laid-Open No. 187590 (1996) or in Japanese Patent Application Laid-Open No. 19892 (1996). However, the invention (alloy) disclosed in these Publications was inferior to alloy H in melting characteristics. Moreover, the aforesaid alloy contains very expensive In and, for this reason, it was difficult to say that the disclosed alloy is a desirable alloy.